A fundamental law of thermodynamics governs lift and therefore the support capacity in the air:Fz=½ρV2SCz in which:Fz represents the lift, in N;ρ represents the density of the fluid, in this case air, in kg/m3;V represents the velocity of the probe system relative to the surrounding air, in m/s;S represents a surface exposed to the airflow, in m2;Cz represents an adimensional lift coefficient.
Conventionally, it is known practice to not measure the density p of the air or the true air speed or TAS, because a simpler method has been used since the beginnings of aviation based on the conventional speed or CAS, the acronym for “Calibrated AirSpeed”, obtained by a difference between a total pressure (stop pressure) supplied by a Pitot probe and a static pressure tapped tangentially to the airflow, considered as close as possible to the ambient pressure in the absence of the aeroplane.
The anemobarometric measurement systems installed on military or civilian aeroplanes exhibit measurements that are likely to be falsified in the case of icing, because of the protuberant nature of the probes used, such as Pitot probes, incidence probes and sideslip probes. Use is therefore conventionally made of devices for de-icing and preventing icing of these probes by reheating, which can result in problems of reliability and lead to a significant electrical consumption.
It is known practice to mitigate these problems by means of flush systems (non-protuberant systems) that for example use:
the parietal technology, i.e. a plurality of local pressure probes (taking a pressure more or less close to the static pressure) distributed over the skin of the aircraft, which, combined with a complex calibration and subject to major installation precautions makes it possible to provide the static pressure, the angle of incidence or AOA, the acronym for “Angle Of Attack”, the conventional speed or CAS, the acronym for “Calibrated AirSpeed”, and the SideSlip Angle, or SSA;
the LiDAR (the acronym for “Light Detection and Ranging”) technology that makes it possible to perform a direct measurement of the air speed outside of the 3D (vectorial) limit layer using laser beams, making it possible to then obtain the True AirSpeed TAS, the Angle Of Attack AOA and the SideSlip Angle SSA;
the technology based on ultrasound probes which provides vector speed information; and
the hybrid technology which combines several of the technologies cited above, for example FR2994273 or FR3035209 thus creating multifunction probes.
These non-protuberant systems exhibit drawbacks of high cost, of installation difficulty because they are generally sensitive to the imperfections of the aeroplane skin, and sometimes of completeness of the flight domain.
Furthermore, in the military field, electromagnetic and thermal stealth are an additional issue, because the reheated protuberant probes exhibit a significant RADAR echo and an infrared-detectable thermal emission.